Field of the Invention
The present invention relates generally to apparatus and methods for removing occluding material from stented regions within blood vessels which have restenosed. More particularly, the present invention relates to apparatus and methods for shearing the occluding material from around an interface envelope defined by the stent.
Percutaneous transluminal angioplasty (PTA) procedures are widely used for treating stenotic atherosclerotic regions of a patient's vasculature to restore adequate blood flow. Catheters having an expansible distal end, usually in the form of an inflatable balloon, are positioned in an artery, for example a coronary artery, at a stenotic site. The expansible end is then expanded to dilate the artery in order to restore adequate blood flow to regions beyond the stenosis. While PTA has gained wide acceptance, it suffers from two major problems: abrupt closure and restenosis.
Abrupt closure refers to rapid reocclusion of the vessel within hours of the initial treatment, and often occurs in patients who have recently suffered acute myocardial infarction. Abrupt closure often results from rapid thrombus formation which occurs in response to injury of the vascular wall from the initial PTA procedure. Restenosis refers to a re-narrowing of the artery over the weeks or months following an initial apparently successful PTA procedure. Restenosis occurs in up to 50% of all PTA patients and results at least in part from smooth muscle cell proliferation and migration.
Many different strategies have been proposed to ameliorate abrupt closure and reduce the restenosis rate. Of particular interest to the present invention, the implantation of vascular stents following PTA has become widespread. Stents are thin-walled tubular scaffolds which are expanded in the arterial lumen following the PTA procedure. Most commonly, the stents are formed from a malleable material, such as stainless steel, and are expanded in situ using a balloon. Alternatively, the stents may be formed from a shape memory alloy or other elastic material, in which case they are delivered in a radially constrained configuration and allowed to self-expand at the PTA treatment site. In either case, the stent acts as a mechanical support for the arterial wall, inhibiting both abrupt closure and restenosis.
While stents have been very successful in inhibiting abrupt closure and reasonably successful in inhibiting restenosis, a significant portion of the treated patient population still experiences restenosis over time. Most stent structures comprise an open lattice, typically in a diamond or a spiral pattern, and cell proliferation (often referred to as hyperplasia) can incur in the interstices between the support elements of the lattice. As a result, instead of forming a barrier to hyperplasia and restenosis, the stent can become embedded within an accumulated mass of thrombus and tissue growth, and the treatment site once again becomes occluded.
To date, proposed treatments for restenosis within previously stented regions of the coronary and other arteries have included both follow-up balloon angioplasty and directional atherectomy, e.g. using the Simpson atherectomy catheter available from Guidant Corporation, Sunnyvale, Calif. Neither approach has been wholly successful. Balloon angioplasty can temporarily open the arterial lumen, but rarely provides long-term patency. Directional atherectomy can successfully debulk the lumen within the stent, but rarely removes the material in a symmetric pattern. Moreover, it has been found that the atherectomy cutting blades can damage the stent, leaving protruding metallic pieces in the blood vessel lumen. Such discontinuities can act as sites for further thrombus formation.
For these reasons, it would be desirable to provide improved methods for treating restenosis within regions of the vasculature which have previously been implanted with stents. More particularly, it would be desirable to provide apparatus and methods for removing stenotic material from within the stents in a uniform and symmetric manner to provide a recanalized vascular lumen which is less likely to suffer from further restenosis. The apparatus and methods will preferably be capable of both dislodging the stenotic material and subsequently capturing and removing the dislodged material from the blood vessel lumen. Desirably, the apparatus will be able to dislodge and remove the stenotic material from along an interface envelope which is defined by the stent which has become embedded within the stenotic material. Removal will preferably be effected in a short amount of time, preferably using only a single or limited number of passes through the restenosed region within the stent. The apparatus and methods will be relatively easy to implement, present acceptable risks to the patient, and be readily performed by physicians who are familiar with balloon angioplasty and other conventional intravascular treatments. At least some of these objectives will be met by the various aspects of the present invention described below.